A fuel cell is being developed as a replacement of a fossil fuel that is not eco-friendly. Differently from a general secondary cell, the fuel cell is for directly converting an energy difference between before and after a reaction generated as hydrogen and oxygen are electrochemically reacted into electric energy without a fuel combustion (oxidation reaction) by supplying a fuel (hydrogen or hydrocarbon) to an anode and supplying oxygen to a cathode.
The fuel cell is being developed to be applied variously as a domestic fuel cell for supplying electricity to a home, a fuel cell used in an electricity car, a fuel cell used in a mobile terminal or a notebook computer, a fuel cell movable at home and supplying electricity, etc.
Especially, a fuel cell for operating home electronics or other electric devices by being moved at home or at an outdoors has to be minimized in order to be conveniently portable, and has to maximize a fuel usage efficiency under a state that the size thereof is limited.
The fuel cell includes a phosphoric acid fuel cell, an alkaline fuel cell, a proton exchange membrane fuel cell(PEMFC), a molten carbonate fuel cell, a solid oxide fuel cell, a direct methanol fuel cell, a direct borohydride fuel cell(DBFC), etc.
As a fuel of the DBFC, KBH4, NaBH4, etc. for decomposing hydrogen are used. In case that NaBH4 is used as a fuel, electrolyte aqueous solution, NaOH or KOH, etc. is added. In case that NaBH4 is used as a fuel and NaOH is used as electrolyte aqueous solution, a following reaction is performed in the anode.2H2O+NaBH4→NaBO2+4H2 
As shown in the above formula, H2 is generated during a power generating process in the fuel cell. The H2 having a great explosive characteristic has to be safely processed. In a fuel efficiency aspect, it is preferable to re-use said H2 by recycling.
Since an amount of a load used at home or in a factory, etc. is variable, it is preferable to control a power generation amount according to the load amount.
However, controlling a power generation amount by controlling a fuel amount or a catalyst amount causes a response time to be late, thereby making a fast reaction impossible. According to this, it is impossible to react to an instantaneous drastic increase of a power thereby not to be able to supply a power stably. Also, in case that a great load is drastically applied in a no-load state, an overshooting phenomenon that a voltage is instantaneously lowered greatly at a unit cell and the lowered voltage is not recovered well is generated more distinctly than a case that a load is gradually increased. Especially, in case that a voltage deviation exists between unit cells at a stack where an anode and a cathode are stacked, a unit cell having a low voltage is greatly damaged by a very low voltage.